Electron accelerator having a coaxial cavity

ABSTRACT

A toroidal cavity-type electron accelerator is provided with a pair of electron beam sources. The beams from the two sources are respectively injected into the accelerating cavity at the mid-plane thereof and at a point displaced from the mid-plane whereby the electrons comprising the beam injected on the mid-plane will be accelerated while the electrons comprising the beam injected at the off mid-plane location will be decelerated and contribute their energy to producing a field for accelerating the electrons of the other beam.

FIELD OF THE INVENTION

The present invention relates to improvements to electron accelerators,and more particularly to electron accelerators having an acceleratingcavity defined by a pair of coaxial conductors.

DESCRIPTION OF THE PRIOR ART

Electron accelerators are generally known, having a resonant cavitysupplied by a high-frequency field source commonly called the HFgenerator, and an electron source capable of injecting these electronsinto the cavity. If certain phase and frequency conditions arerespected, these electrons are accelerated by the electric fieldthroughout their passage through the cavity.

These are in general machines working in the pulsed regime, and havingrelatively low beam intensities.

In document WO-A-88/09597 (Atomic Energy Commission), an electronaccelerator with recirculation, of novel design, was proposed.

This document describes an electron accelerator which is characterizedin that the resonant cavity is a coaxial cavity defined by an outercylindrical conductor and an inner cylindrical conductor having the sameaxis. The electron beam is injected into this cavity in the mid planewhich is perpendicular to the axis, along a first diameter. An electrondeflector makes it possible to deflect the beam once it has passedthrough the cavity a first time, and reinject it back into the cavitywhere it undergoes a second acceleration, etc.

The above-described prior art device is also called a "rhodotron"because the electron beam passes through the cavity several times alonga trajectory which describes the pattern of the petals of a flower.

The rhodotron has several advantages, namely that its shape isparticularly simple and compact. In addition, the principle according towhich the device functions makes it possible to obtain an intense andcontinuous beam, which was not the case with conventional devicesworking in the pulsed regime.

Furthermore, the rhodotron described is self-focusing due to the factthat the magnetic deflectors, which have input phases in the shape ofvery wide dihedra, provide suitable focusing of the electron beam. It isconsequently not necessary to provide additional focusing elements.

Finally, the electron beam injected in the mid-plane of the rhodotron isnot deviated. This is because the beam is not subjected to the magneticfield, which is zero in the mid-plane according to the configurationdescribed in WO-A-88/09597.

However, a rhodotron requires the cavity to be supplied by ahigh-frequency field source. In particular, in the device described inWO-A-88/09597, an electric field of several hundreds of megahertz isgenerated by an external high-frequency generator.

High-frequency generators an output power of approximately 200 kW, whichcan create the requisite electric fields of several hundreds ofmegahertz, are relatively expensive devices. Such generators essentiallyuse electron tubes of the triode, tetrode or pentode type, and useadvanced, therefore expensive, techniques such as metal/ceramic welding,the use of refractory material grids or the use of thoriated tungstenfilaments.

U.S. Pat. No. 4,763,079 describes a method for decelerating a particlebeam, in which the energy produced by the deceleration of the particlesis stored in order to be used for accelerating electrons in anotheraccelerator.

OBJECT OF THE INVENTION

The object of the present invention is to provide a device which makesit possible to avoid the use of particularly expensive high-frequencygenerators, whilst retaining the advantages intrinsic to the originalarrangement of the electron accelerator of the type described indocument WO-A-88/09597.

SUMMARY OF THE INVENTION

The present invention relates to an electron accelerator, comprising:

a first source emitting an electron beam to be accelerated,

a coaxial cavity defined by an outer cylindrical conductor and an innercylindrical conductor, of the same axis, the electron beam beinginjected in the mid-plane which is perpendicular to the axis, along afirst diameter of the outer conductor,

the accelerator being characterized in that it includes a second sourceemitting an electron beam, this electron beam being decelerated when itpasses through the coaxial cavity, making it possible to produce theelectromagnetic field necessary for accelerating the electron beam fromthe first source.

The second electron beam is injected into the coaxial cavity along aplane which is different from the mid-plane, which makes it possible todeflect the electrons towards the walls of the cavity and to remove themfrom this cavity.

The second electron beam source is provided with a device making itpossible to modulate the intensity of the electrons emitted, inparticular a control grid or a rearranger. Such devices are well knownin devices which employ electron beams. The intensity of the electronbeam is modulated such that the electrons from the second source appearin the cavity at the moment when they encounter a decelerating radialelectric field. In this way, the electrons give up their kinetic energyto the electromagnetic field in the cavity and establish and maintainthe electromagnetic field. The energy of the electrons injected by thesecond source is preferably chosen so that these electrons reach thewall of the cavity with a low but non-zero residual energy. In this way,the energy conversion between the electron beam and the cavity can reachvalues of 80 to 90%.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 represents a section along the mid-plane of an accelerator inaccordance with the present invention, the accelerator having a coaxialcavity.

FIG. 2 represents a cross-sectional view of the accelerator of FIG. 1,FIG. 2 being a view taken parallel to the principal axis of the coaxialcavity of an electron accelerator according to the present invention.

DESCRIPTION OF A PARTICULAR EMBODIMENT OF THE PRESENT INVENTION

FIG. 1 represents a section along the mid-plane of the coaxial cavity ofthe electron accelerator according to the present invention.

The cavity 5 is defined by an outer cylindrical conductor 10 and aninner cylindrical conductor 20, of the same axis, and two flanges 15 and25 (see FIG. 2) which are oriented perpendicularly with respect to thecommon axis 30 of the conductors.

According to this configuration, the electric field E is purely radial;it is maximum in the mid-plane 40 and decreases on either side of thisplane to vanish at the flanges 15 and 25. Similarly, the magnetic fieldM is maximum along the flanges and vanishes in the mid-plane whilechanging polarity.

The principal electron beam 1 is injected from a source 100 into thecoaxial cavity 5 along the mid-plane 40, and is not deflected becausethe magnetic field M at the point of injection is equal to zero.

The electron beam 1 penetrates into the cavity through an aperture 11along a first diameter of the outer conductor 10; it traverses the innerconductor 20 by passing through two diametrically opposite apertures 21and 22 and leaves the cavity through an aperture 12.

If certain phase and frequency conditions are satisfied, the principalbeam 1 will be accelerated over its entire passage through the coaxialcavity 5.

In particular, it is suitable for the electric field E to vanish, i.e.,to change polarity, when the beam passes through the inner conductor 20,so that the field causes acceleration during passage through the firstpart of the cavity (between the outer conductor 10 and the innerconductor 20), and again causes acceleration, being therefore opposite,during passage over the second part of the trajectory, that is to saybetween the inner conductor 20 and the outer conductor 10.

At least one deflector 51 is arranged outside the coaxial cavity 5.Deflector 51 deflects the principal electron beam 1 and reinjects italong a second diameter of the outer conductor 10. The principal beam isreintroduced to cavity 5 via an aperture 13 where it again undergoesacceleration and re-emerges through the aperture 14.

An electron beam exiting cavity 5 via aperture 14 may be again deflectedby a deflector 53 and reinjected along a third diameter into the cavity,where it will undergo a third acceleration, etc.

The magnetic deflectors 51, 53, . . . advantageously have input faces inthe shape of a very wide dihedron, so as to focus the principal electronbeam 1.

FIG. 2 represents a cross-sectional view taken perpendicular to FIG. 1,i.e., in a direction parallel to the principal axis of the coaxialcavity.

According to the principal characteristic of the present invention, theelectron accelerator having a coaxial cavity includes a second electronbeam source 200 which is provided with a device 210 for modulating thebeam intensity. Source 200 emits an electron beam 2 which will beinjected into the cavity 5 at the moment when the electric field Ecauses its deceleration. This makes it possible to generate theelectromagnetic field necessary for accelerating the first electron beam1.

The kinetic energy loss of the electron which is decelerated makes itpossible to create a high-frequency electromagnetic field in the coaxialcavity 5.

Preferably, the second electron beam 2 is injected into the coaxialcavity 5 along a plane which is different from the mid-plane 40. Theresult of this is that the electrons comprising beam 2 will be deflectedtowards the walls of the cavity, which allows them to be removed fromthe cavity.

The electrons comprising beam 2 are not slowed to rest in the cavityitself thereby ensuring that the electrons will not be subjected, in theopposite direction, to the acceleration of the electromagnetic field andreaccelerated.

Restated, it is necessary for the electrons from the secondary beam 2still to have some degree of residual kinetic energy, so as to reach thewalls of the cavity 5.

Because of this, the degree of conversion of the kinetic energy of theelectrons into electromagnetic energy is limited to values of from 80 to90%.

The present invention makes it possible not to have to resort to usingexternal high-frequency generators, which are particularly expensivedevices. In fact, they represent approximately 30% of the total cost ofan electron accelerator.

Furthermore, the structure of an accelerator according to the presentinvention is simplified, which provides a non-negligible improvement inthe reliability of the electron accelerator.

I claim:
 1. An electron accelerator comprising:a first source forgenerating a first beam of electrons to be accelerated; a cavity definedby an outer cylindrical conductor and an inner cylindrical conductor,said conductors being coaxial and being joined by flanges at theiropposite ends, said cavity having a mid-plane disposed substantiallyequidistantly from said flanges, said mid-plane being orientedperpendicular with respect to the axis of said conductors, said firstelectron beam being injected into said cavity at said mid-plane along afirst diameter of said outer conductor; a second source for generating asecond beam of electrons, said second electron beam being deceleratedwhen it passes through said cavity to thereby provide an electromagneticfield necessary for accelerating said first electron beam.
 2. Theelectron accelerator according to claim 1, characterized in that saidsecond electron beam is injected into said cavity along a plane which isdifferent from said mid-plane, thus allowing the electrons comprisingsaid second beam to be deflected towards the walls of said cavity. 3.The electron accelerator according to claim 1, characterized in thatsaid second electron beam source includes means for modulating theintensity of said second electron beam.
 4. The electron acceleratoraccording to claim 1, further comprising at least one electron deflectorpositioned outside of said cavity, said one deflector intercepting saidfirst electron beam after passage through said cavity along said firstdiameter and deflecting said first beam, said deflected first beam beingreinjected into said cavity at said mid-plane along a second diameter ofsaid outer conductor.
 5. The electron accelerator according to claim 2,characterized in that said second electron beam source includes meansfor modulating the intensity of said second electron beam.
 6. Theelectron accelerator according to claim 2, further comprising at leastone electron deflector positioned outside of said cavity, said onedeflector intercepting said first electron beam after passage throughsaid cavity along said first diameter and deflecting said first beam,said deflected first beam being reinjected into said cavity at saidmid-plane along a second diameter of said outer conductor.
 7. Theelectron accelerator according to claim 3, further comprising at leastone electron deflector positioned outside of said cavity, said onedeflector intercepting said first electron beam after passage throughsaid cavity along said first diameter and deflecting said first beam,said deflected first beam being reinjected into said cavity at saidmid-plane along a second diameter of said outer conductor.